1. Field of the Invention
The present invention relates to an optical fiber suitable for transmitting signals of a plurality of channels in wavelength division multiplexing (WDM) transmission, a method of making the same, and an optical transmission system including the same.
2. Related Background Art
WDM transmission enables high-speed, large-capacity optical communications by transmitting signals of a plurality of channels (hereinafter referred to as WDM signals). Silica type optical fibers are employed as an optical transmission line for WDM transmission since their transmission loss is low in the vicinity of a wavelength of 1.55 xcexcm, whereas WDM signals in the wavelength band of 1.55 xcexcm are utilized since optical amplifiers enabling signal amplification in the 1.55-xcexcm wavelength band have been put into practice.
If chromatic dispersion occurs in a signal wavelength region (e.g., 1.55-xcexcm wavelength band), then the pulse waveform of each WDM signal deforms, whereby transmission characteristics deteriorate. Therefore, from the viewpoint of preventing the signal waveform from deteriorating, the chromatic dispersion in optical transmission lines for transmitting WDM signals is desired to be low in the signal wavelength region. If the chromatic dispersion in the signal wavelength region is substantially zero, on the other hand, then four-wave mixing, which is a kind of nonlinear optical phenomena, is likely to occur, thereby causing crosstalk and noise, by which transmission characteristics deteriorate. For suppressing the occurrence of four-wave mixing, the repeating interval may be shortened so as to lower the signal power. However, this requires a number of optical amplifiers to be disposed in an optical transmission line, whereby thus obtained optical transmission system becomes expensive as a whole.
For example, in order to deal with the foregoing problems, U.S. Pat. No. 5,894,537 proposes a dispersion management optical fiber in which a plurality of positive and negative dispersion segments having positive and negative chromatic dispersions, respectively, at a predetermined wavelength (e.g., wavelength of 1.55 xcexcm) are alternately disposed along the longitudinal direction. If such a dispersion management optical fiber is employed in an optical transmission line, then the average chromatic dispersion in the optical transmission line as a whole becomes substantially zero, whereby transmission characteristics are effectively restrained from deteriorating due to cumulative chromatic dispersion. Since a positive or negative chromatic dispersion occurs (the absolute value of chromatic dispersion is not near zero) in substantially all the regions of the optical transmission line, transmission characteristics are restrained from deteriorating due to four-wave mixing.
The inventors studied the prior art mentioned above and, as a result, have found a problem as follows. Namely, signal pulses may deteriorate their waveforms even in an optical transmission system in which a dispersion management optical fiber is employed in an optical transmission line as mentioned above, whereby there has been a possibility of high-speed, large-capacity optical communications failing to be realized in such a case.
In order to overcome the problem mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure making it possible to realize higher-speed, larger-capacity optical communications as compared with those with conventional optical transmission systems, a method of making the same, and an optical transmission system including the same.
The optical fiber according to the present invention constitutes at least a part of any repeated transmission line between a transmitting station for sending out signals of a plurality of channels and a repeater station including an optical amplifier or the like, between repeater stations, and between a repeater station and a receiving station. Also, this optical fiber is an optical fiber mainly composed of silica glass, in which at least one first portion having a positive chromatic dispersion at a predetermined wavelength in a wavelength band in use and at least one second portion having a negative chromatic dispersion at the predetermined wavelength are arranged adjacent each other along the longitudinal direction. While the first and second portions having positive and negative chromatic dispersions, respectively, are disposed alternately in the longitudinal direction so as to effect dispersion management, the optical fiber according to the present invention further comprises a polarization coupling structure for inducing coupling between polarization modes of propagating light. This optical fiber may be either a unitary optical fiber in which the first and second portions are alternately arranged along the longitudinal direction thereof or an optical fiber in which optical fibers having positive and negative chromatic dispersions corresponding to the first and second portions, respectively, are alternately fusion-spliced.
Consequently, the optical fiber lowers each of the deterioration in signal waveform caused by nonlinear optical phenomena and the deterioration in signal waveform caused by cumulative chromatic dispersion. Also, the polarization coupling structure for inducing coupling between polarization modes of propagating light lowers the deterioration in waveform caused by polarization mode dispersion. Therefore, the optical fiber according to the present invention is excellent in transmission quality, and enables higher-speed, larger-capacity WDM transmission.
In the optical fiber according to the present invention, the polarization mode dispersion at the predetermined wavelength is preferably 0.1 psxc2x7kmxe2x88x92xc2xd or less. Also, its average coupling length, which is defined by the length from an end part of the optical fiber to a boundary at which a region where the polarization mode dispersion value is in proportion to the fiber length shifts to a region where the polarization mode dispersion value is in proportion to the square root of fiber length, is preferably 10 m or less. In each of these cases, the deterioration in waveform caused by polarization mode dispersion is sufficiently lowered.
The above-mentioned polarization coupling structure includes a twist provided to the optical fiber. In this case, thus provided twist induces random coupling between polarization modes of propagating light, thereby lowering the deterioration in waveform caused by polarization mode dispersion. In particular, the average rate of twist is preferably 2 rotations/m or more, whereas the deviation of twist rate is preferably 1 rotation/m or more. Further, a transient region disposed between each of first portion having a positive chromatic dispersion and each second portion having a negative chromatic dispersion is preferably not substantially provided with the above-mentioned twist. This is because of the fact that the controllability of drawing conditions improves in the transient region, whereby the longitudinal change of chromatic dispersion can be stabilized.
In a method of making an optical fiber comprising the characteristic structure as explained in the foregoing, an optical fiber preform is prepared at first, and a part of thus prepared optical fiber preform is drawn while being heated. In particular, a drawing condition, for example, such as the fiber diameter or the drawing tension, is changed at the time of drawing such that at least one first portion having a positive chromatic dispersion at a predetermined wavelength within the wavelength band in use and at least one second portion having a negative dispersion at the predetermined wavelength are alternately formed along the longitudinal direction. In addition, this drawing provides a predetermined portion of thus obtained optical fiber with a twist. By way of such a manufacturing process, the optical fiber comprising a characteristic structure as mentioned above is obtained.
The average rate of the above-mentioned twist is preferably 2 rotations/m or more, whereas the deviation of the twist rate is preferably 1 rotation/m or more. Preferably, the transient region positioned between the first and second portions in the optical fiber obtained by the above-mentioned making method is not substantially provided with the above-mentioned twist. This is because of the fact that the controllability of drawing conditions improves in the transient region of the resulting optical fiber, and the longitudinal change of chromatic dispersion is stabilized.
Further, the optical transmission system according to the present invention comprises, as at least a part of an optical transmission line in a repeated section, an optical fiber comprising the characteristic structure as mentioned above. This lowers the waveform deterioration caused by any of nonlinear optical phenomena, cumulative chromatic dispersion, and polarization mode dispersion, and enables higher-speed, larger-capacity WDM transmission excellent in transmission quality.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.